Abstract:

A method for producing boehmite particles includes subjecting powder of
aluminum hydroxide to hydrothermal reaction together with a nucleation
agent, thereby obtaining boehmite particles having an average primary
particle size of 0.6 μm or less and including primary particles each
having a hexahedral shape. A method for producing alumina particles
includes: drying the boehmite particles produced by the above described
method; calcining the boehmite particles, which have been dried, to
obtain alumina particles; and disintegrating the obtained alumina
particles.

Claims:

1. A method for producing boehmite particles, comprising subjecting powder
of aluminum hydroxide to hydrothermal reaction together with a nucleation
agent, thereby obtaining boehmite particles having an average primary
particle size of 0.6 μm or less and including primary particles each
having a hexahedral shape.

2. The method according to claim 1, wherein the powder of aluminum
hydroxide is gibbsite.

3. The method according to claim 1, wherein the nucleation agent is metal
salt or sol of metal oxide.

4. The method according to claim 1, wherein the step of subjecting powder
of aluminum hydroxide to hydrothermal reaction together with a nucleation
agent is performed by subjecting slurry containing the powder of aluminum
hydroxide and the nucleation agent to the hydrothermal reaction, the
slurry having a pH of 8 or lower.

5. The method according to claim 1, wherein the step of subjecting powder
of aluminum hydroxide to hydrothermal reaction together with a nucleation
agent is performed by subjecting slurry containing the powder of aluminum
hydroxide and the nucleation agent to the hydrothermal reaction, the
slurry having an electrical conductivity of 500 μS/cm or lower.

6. The method according to claim 1, further comprising a step of
collecting, as filtrate, a water phase in slurry obtained as a result of
the hydrothermal reaction and containing the boehmite particles while
introducing pure water into the slurry and until the electrical
conductivity of the collected filtrate becomes 50 μS/cm or lower.

7. A method for producing alumina particles, comprising:drying boehmite
particles produced by subjecting powder of aluminum hydroxide to
hydrothermal reaction together with a nucleation agent, the boehmite
particles having an average primary particle size of 0.6 μm or less
and including primary particles each having a hexahedral shape;calcining
the boehmite particles, which have been dried, to obtain alumina
particles; anddisintegrating the obtained alumina particles.

Description:

BACKGROUND OF THE INVENTION

[0001]The present invention relates to a method for producing boehmite
particles which can be used as a raw material for the production of
alumina particles, and a method for producing alumina particles.

[0002]Alumina particles are used, for instance, as abrasive grains in an
application of polishing an object to be polished, such as a
semiconductor device substrate, a substrate for a display, a hard disk
substrate, and a sapphire substrate for an LED. In order to obtain a
polished surface with high smoothness and few defects, the particle size
of the alumina particles to be used as abrasive grains is preferably as
small as possible. A polishing composition which contains alumina
particles as free abrasive grains generally has a higher removal rate
(polishing rate) against the object to be polished than a polishing
composition which contains colloidal silica as free abrasive grains.
However, even in the case of a polishing composition containing alumina
particles, the removal rate by the polishing composition against the
object to be polished is not particularly high when the alumina particles
have relatively small particle sizes. In addition, when the alumina
particles have relatively small particle sizes, it is difficult to remove
the alumina particles that have deposited on the polished surface (in
other words, ease of washing off the alumina particles on the polished
surface is not high).

[0003]As is described in Japanese Laid-Open Patent Publication No.
3-277683 and Japanese Laid-Open Patent Publication No. 5-271647, it is
known to use alumina particles including primary particles each having an
angular shape as abrasive grains. Alumina particles including primary
particles each having a hexahedral shape, which are conceptually included
in the angular alumina particles, are capable of polishing an object to
be polished with a high removal rate and are easily washed from the
surface of the object after polishing even when their particle sizes are
relatively small. Accordingly, such alumina particles are suitable as
abrasive grains in an application of polishing an object to be polished
so as to obtain a polished surface with high smoothness and few defects.

SUMMARY OF THE INVENTION

[0004]Accordingly, an objective of the present invention is to provide a
method suitable for producing alumina particles which can be used
successfully as abrasive grains in an application of polishing an object
to be polished so as to obtain a polished surface with high smoothness
and few defects, and a method suitable for producing boehmite particles
which are useful as a raw material for such alumina particles.

[0005]In order to achieve the above described objective, according to one
aspect of the present invention, a method for producing boehmite
particles is provided, which method includes subjecting powder of
aluminum hydroxide to hydrothermal reaction together with a nucleation
agent, thereby obtaining boehmite particles having an average particle
size of 0.6 μm or less and including primary particles each having a
hexahedral shape.

[0006]The powder of aluminum hydroxide to be used is preferably gibbsite.
The nucleation agent to be used is preferably metal salt or sol of metal
oxide.

[0007]The step of subjecting powder of aluminum hydroxide to hydrothermal
reaction together with a nucleation agent is performed by subjecting, for
instance, slurry containing the powder of aluminum hydroxide and the
nucleation agent to the hydrothermal reaction. The pH of the slurry is
preferably 8 or lower. The electrical conductivity of the slurry is
preferably 500 μS/cm or lower.

[0008]The method for producing boehmite particles preferably further
includes collecting, as filtrate, a water phase in slurry obtained as a
result of the hydrothermal reaction and containing the boehmite particles
while introducing pure water into the slurry and until the electrical
conductivity of the collected filtrate becomes 50 μS/cm or lower.

[0009]According to another aspect of the present invention, a method for
producing alumina particles is provided, which method includes: drying
boehmite particles produced by the above described method; calcining the
boehmite particles after having been dried to obtain alumina particles;
and disintegrating the obtained alumina particles.

[0010]Other aspects and advantages of the invention will become apparent
from the following description illustrating by way of example the
principles of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0011]One embodiment of the present invention will now be described below.

[0012]At first, before a method for producing boehmite particles according
to the present embodiment will be described, boehmite particles produced
by the method according to the present embodiment will now be described.
The main application of boehmite particles produced by the method
according to the present embodiment is as a raw material for the
production of alumina particles used as abrasive grains.

[0013]Boehmite particles produced by the method according to the present
embodiment include primary particles each having a hexahedral shape. Each
primary particle of the boehmite particles preferably has an outer shape
similar to a parallelepiped defined by two square surfaces that face each
other and four rectangular or square surfaces, or a parallelepiped
defined by two rhombic surfaces that face each other and four rectangular
or square surfaces. The shapes of the primary particles of alumina
particles obtained by calcining boehmite particles usually inherit the
almost shapes of the primary particles of the boehmite particles, so that
it can be said that when boehmite particles including primary particles
each having a hexahedral shape are used, alumina particles including
primary particles each having a hexahedral shape are easily obtained.
Alumina particles including primary particles each having a hexahedral
shape are suitable as abrasive grains in an application of polishing an
object to be polished so as to obtain a polished surface with high
smoothness and few defects. Boehmite particles including primary
particles each having a hexahedral shape are useful at least as a raw
material for such alumina particles.

[0014]The primary particles of boehmite particles produced by the method
according to the present embodiment have an average particle size of 0.6
μm or less. In other words, boehmite particles produced by the method
according to the present embodiment have an average primary particle size
of 0.6 μm or less. The average primary particle size of the boehmite
particles is preferably 0.4 μm or less, more preferably 0.3 μm or
less, and further preferably 0.2 μm or less. The average primary
particle size of boehmite particles including primary particles each
having a hexahedral shape is defined as an average length of sides each
of which is the longest among three sides extending from one vertex of
each primary particle of the boehmite particles. The value of the average
primary particle size of alumina particles obtained by calcining boehmite
particles usually inherits the almost value of the average primary
particle size of the boehmite particles, so that it can be said that when
boehmite particles having an average primary particle size in any of the
above described ranges are used, alumina particles having an average
primary particle size in the range are easily obtained. Alumina particles
having an average primary particle size of 0.6 μm or less, or more
specifically 0.4 μm or less, 0.3 μm or less, or 0.2 μm or less
are suitable as abrasive grains in an application of polishing an object
to be polished so as to obtain a polished surface with high smoothness
and few defects. Boehmite particles having an average primary particle
size of 0.6 μm or less, or more specifically 0.4 μm or less, 0.3
μm or less, or 0.2 μm or less are useful at least as a raw material
for such alumina particles.

[0015]The average primary particle size of boehmite particles produced by
the method according to the present embodiment is preferably 0.01 μm
or more, more preferably 0.05 μm or more, further preferably 0.08
μm or more, and particularly preferably 0.1 μm or more. As
described above, the value of the average primary particle size of
alumina particles obtained by calcining boehmite particles usually
inherits the almost value of the average primary particle size of the
boehmite particles, so that it can be said that when boehmite particles
having an average primary particle size in any of the above described
ranges are used, alumina particles having an average primary particle
size in the range are easily obtained. As the average primary particle
size of alumina particles is larger, the removal rate (polishing rate) by
the alumina particles against an object to be polished is enhanced. In
this regard, when the average primary particle size of alumina particles
is 0.01 μm or more, or more specifically 0.05 μm or more, 0.08
μm or more, or 0.1 μm or more, the removal rate is easily enhanced
up to a level particularly suitable for practical use. Boehmite particles
having an average primary particle size of 0.01 μm or more, or more
specifically 0.05 μm or more, 0.08 μm or more, or 0.1 μm or more
are useful at least as a raw material for such alumina particles.

[0016]The primary particles of boehmite particles produced by the method
according to the present embodiment have an aspect ratio preferably in a
range of 1 to 5, more preferably of 1 to 3, further preferably of 1 to 2,
and particularly preferably of 1 to 1.5. The aspect ratio of boehmite
primary particles each having a hexahedral shape is defined as an average
of values each obtained by dividing a by c, where the length of the
longest side is represented by a and the length of the shortest side is
represented by c among three sides extending from one vertex of each
boehmite primary particle. The length of the remaining one side b of each
boehmite primary particle is preferably approximately equal to the length
of the longest side a of the boehmite primary particle. The aspect ratio
of the primary particles of alumina particles obtained by calcining
boehmite particles usually inherits the almost aspect ratio of the
primary particles of the boehmite particles, so that it can be said that
when boehmite particles including primary particles having an aspect
ratio in any of the above described ranges are used, alumina particles
including primary particles having an aspect ratio in the range are
easily obtained. Alumina particles including primary particles having an
aspect ratio of 1 to 5, or more specifically 1 to 3, 1 to 2, or 1 to 1.5
are particularly suitable as abrasive grains in an application of
polishing an object to be polished so as to obtain a polished surface
with high smoothness and few defects. Boehmite particles including
primary particles having an aspect ratio of 1 to 5, or more specifically
1 to 3, 1 to 2, or 1 to 1.5 are useful at least as a raw material for
such alumina particles.

[0017]A method for producing boehmite particles according to the present
embodiment includes using powder of aluminum hydroxide as a starting
material and subjecting the powder to hydrothermal reaction. Powder of
aluminum hydroxide suitable as the starting material is gibbsite.

[0018]Powder of aluminum hydroxide used as the starting material has an
average primary particle size of preferably 10 μm or less. As the
average primary particle size of powder of aluminum hydroxide is smaller,
the efficiency of the hydrothermal reaction increases. In this regard,
when the average primary particle size of powder of aluminum hydroxide is
10 μm or less, the efficiency of the hydrothermal reaction is easily
enhanced up to a level particularly suitable for practical use.

[0019]Boehmite particles are produced (synthesized) by subjecting powder
of aluminum hydroxide to hydrothermal reaction together with a nucleation
agent. The nucleation agent refers to an auxiliary agent which forms
micronuclei in water when subjected to hydrothermal reaction together
with powder of aluminum hydroxide. The micronuclei formed by the
nucleation agent act as nuclei, from which boehmite particles grow. A
preferable nucleation agent is sol of metal oxide, or metal salt. Among
them, alumina sol or aluminum nitrate (aluminum nitrate nonahydrate),
which are the same aluminum compound as boehmite, are more preferable.
However, when alumina sol is used as the nucleation agent, boehmite
particles having small primary particle sizes are more easily obtained
than the case of having used aluminum nitrate. In this point, alumina sol
is particularly preferable.

[0020]Alumina sol used as the nucleation agent has an average particle
size of preferably 5 to 30 nm, and more preferably 5 to 15 nm. When
alumina sol having an average particle size in any of the above described
ranges is used, the size of boehmite particles obtained is easily
controlled. The average particle size of alumina sol is equal to the
particle size of the last cumulative particle when the cumulative volume
of particles in the alumina sol in order from the smallest particle
becomes 50% or more of the total volume of all the particles in the
alumina sol, and can be measured by using, for instance, a dynamic light
scattering type nanotrack particle size distribution measurement
instrument (UPA series) made by NIKKISO CO., LTD.

[0021]Alumina sol used as the nucleation agent has a largest particle size
of preferably 200 nm or less. When alumina sol having a largest particle
size of 200 nm or less is used, the size of boehmite particles obtained
is easily controlled. The largest particle size of alumina sol is equal
to the particle size of the particle having the largest size in the
alumina sol, and can be measured by using, for instance, AccuSizer model
780 made by Particle Sizing Systems.

[0022]The amount of a nucleation agent used in the hydrothermal reaction
is preferably 0.000001 parts by weight or more, more preferably 0.00001
parts by weight or more, further preferably 0.0001 parts by weight or
more, and particularly preferably 0.001 parts by weight or more when the
amount of powder of aluminum hydroxide used in the hydrothermal reaction
is 100 parts by weight. As the amount of a nucleation agent used
increases, boehmite particles having small primary particle sizes are
easily obtained. In this regard, when the amount of a nucleation agent
used is 0.000001 parts by weight or more, or more specifically 0.00001
parts by weight or more, 0.0001 parts by weight or more, or 0.001 parts
by weight or more if the amount of powder of aluminum hydroxide used is
100 parts by weight, boehmite particles having an average primary
particle size of 0.6 μm or less are particularly easily obtained
practically.

[0023]In addition, the amount of a nucleation agent used is preferably 10
parts by weight or less, more preferably 1 part by weight or less,
further preferably 0.5 parts by weight or less, and particularly
preferably 0.1 parts by weight or less when the amount of powder of
aluminum hydroxide used is 100 parts by weight. As the amount of a
nucleation agent used decreases, boehmite particles having large primary
particle sizes are easily obtained. In this regard, when the amount of a
nucleation agent used with respect to 100 parts by weight of the powder
of aluminum hydroxide used is 10 parts by weight or less, or more
specifically 1 part by weight or less, 0.5 parts by weight or less, or
0.1 parts by weight or less if the amount of powder of aluminum hydroxide
used is 100 parts by weight, boehmite particles having an average primary
particle size of 0.01 μm or more are particularly easily obtained
practically.

[0024]The above described hydrothermal reaction is carried out, for
instance, by exposing slurry which is obtained by dispersing or
dissolving the powder of aluminum hydroxide and the nucleation agent in
water to high temperature and high pressure. An autoclave is suitable for
the operation.

[0025]The above described slurry contains powder of aluminum hydroxide in
an amount of preferably 1% by mass or more, and more preferably 10% by
mass or more. As the content of powder of aluminum hydroxide in the
slurry increases, the synthesis efficiency of boehmite particles in the
hydrothermal reaction increases. In this regard, when the content of
powder of aluminum hydroxide in the slurry is 1% by mass or more, or more
specifically 10% by mass or more, the synthesis efficiency of boehmite
particles in the hydrothermal reaction is easily enhanced up to a level
particularly suitable for practical use.

[0026]In addition, the slurry contains powder of aluminum hydroxide in an
amount of preferably 30% by mass or less, and more preferably 20% by mass
or less. As the content of powder of aluminum hydroxide in the slurry
decreases, the yield of boehmite particles including primary particles
each having a hexahedral shape in the hydrothermal reaction increases. In
this regard, when the content of powder of aluminum hydroxide in the
slurry is 30% by mass or less, or more specifically 20% by mass or less,
the yield of boehmite particles including primary particles each having a
hexahedral shape in the hydrothermal reaction is easily enhanced up to a
level particularly suitable for practical use.

[0027]The slurry has a pH of preferably 8 or lower, more preferably 7 or
lower, and further preferably 6 or lower. As the pH of the slurry is
lowered, boehmite particles having a small aspect ratio are easily
obtained in the hydrothermal reaction. In this regard, when the pH of the
slurry is 8 or lower, or more specifically 7 or lower, or 6 or lower,
boehmite particles having an aspect ratio of 5 or smaller are
particularly easily obtained practically. However, when a reaction
container used for the hydrothermal reaction is made of metal, the slurry
has a pH preferably 2 or more in order to prevent the corrosion of the
reaction container. A method for adjusting the pH of the slurry is not
particularly limited, but includes, for instance, an addition of an
inorganic acid such as nitric acid to the slurry, ion exchange treatment,
decantation, and cross flow filtration with the use of an ultrafiltration
membrane. These methods may be used solely or in combination with another
method.

[0028]The slurry has an electrical conductivity of preferably 500 μS/cm
or lower, more preferably 300 μS/cm or lower, further preferably 200
μS/cm or lower, and particularly preferably 100 μS/cm or lower. As
the electrical conductivity of the slurry is lowered, boehmite particles
having small primary particle sizes becomes easier to obtain in the
hydrothermal reaction. In this regard, when the electrical conductivity
of the slurry is 500 μS/cm or lower, or more specifically 300 μS/cm
or lower, 200 μS/cm or lower, or 100 μS/cm or lower, boehmite
particles having an average primary particle size of 0.6 μm or less
are particularly easily obtained practically. A method for adjusting the
electrical conductivity of the slurry is not particularly limited, but
includes, for instance, ion exchange treatment, decantation, and cross
flow filtration with the use of an ultrafiltration membrane. These
methods may be used solely or in combination with another method.

[0029]A temperature of the hydrothermal reaction is preferably 180°
C. or higher, more preferably 185° C. or higher, and further
preferably 190° C. or higher. As a temperature of the hydrothermal
reaction is raised, the synthesis efficiency of boehmite particles in the
hydrothermal reaction increases. In this regard, when a temperature of
the hydrothermal reaction is 180° C. or higher, or more
specifically 185° C. or higher, or 190° C. or higher, the
synthesis efficiency of boehmite particles is easily enhanced up to a
level particularly suitable for practical use.

[0030]In addition, a temperature of the hydrothermal reaction is
preferably 300° C. or lower, more preferably 210° C. or
lower, and further preferably 205° C. or lower. As a temperature
of the hydrothermal reaction is lowered, boehmite particles having
uniform primary particle sizes are easily obtained in the hydrothermal
reaction. In this regard, when a temperature of the hydrothermal reaction
is 300° C. or lower, or more specifically 210° C. or lower,
or 205° C. or lower, boehmite particles having uniform primary
particle sizes are particularly easily obtained practically.

[0031]A pressure of the hydrothermal reaction is not particularly limited.
Boehmite particles can be favorably obtained by using an autogenous
pressure, but an increased pressure or a reduced pressure may be
employed, as needed.

[0032]A period of time for the hydrothermal reaction is not particularly
limited, but it is difficult to obtain a sufficient amount of boehmite
particles in an excessively short period of time. A preferable period of
time for the hydrothermal reaction is, for instance, 8 hours or longer
when a temperature of the hydrothermal reaction is 180° C., 6
hours or longer when a temperature of the hydrothermal reaction is
190° C., 4 hours or longer when a temperature of the hydrothermal
reaction is 195° C., and 3 hours or longer when a temperature of
the hydrothermal reaction is 200° C.

[0033]Boehmite particles obtained in the hydrothermal reaction are
preferably cleaned. Powder of aluminum hydroxide often contains sodium
ions as impurities, so that the sodium ions will remain also in boehmite
particles as impurities, which are synthesized through a hydrothermal
reaction with the use of such powder of aluminum hydroxide as a raw
material. Impurity sodium ions in boehmite particles is not desirable,
depending on an application of the boehmite particles. Accordingly,
boehmite particles obtained in the hydrothermal reaction are cleaned when
the sodium ions contained in the boehmite particles needs to be removed.
A typical method for cleaning boehmite particles includes, for instance,
methods of using a rotary filter, and cross flow filtration with the use
of an ultrafiltration membrane. In these methods, boehmite particles are
cleaned by collecting, as filtrate, a water phase in slurry containing
the boehmite particles while introducing pure water in the slurry.

[0034]The amount of sodium ions which are contained in boehmite particles
as impurities is positively correlated to the electrical conductivity
which is measured for filtrate collected in a process of cleaning the
boehmite particles. Accordingly, the electrical conductivity of filtrate
collected in a process of cleaning boehmite particles can be an indicator
for the amount of impurity sodium ions in the boehmite particles.
Boehmite particles which are assumed to be used as a raw material for
alumina particles are cleaned until the electrical conductivity of
filtrate collected in the cleaning process becomes preferably 50 μS/cm
or lower, more preferably becomes 40 μS/cm or lower, and further
preferably becomes 20 μS/cm or lower. As the value of the electrical
conductivity of filtrate collected in the cleaning process is smaller,
the alpha ratio of alumina particles obtained by calcining the boehmite
particle is easily controlled. In this regard, when boehmite particles
are cleaned until the electrical conductivity of filtrate collected in
the cleaning process becomes 50 μS/cm or lower, or more specifically
40 μS/cm or lower, or 20 μS/cm or lower, the controllability for
the alpha ratio of alumina particles can be enhanced up to a level
particularly suitable for practical use.

[0035]Next, before a method for producing alumina particles according to
the present embodiment will be described, alumina particles produced by
the method according to the present embodiment will now be described. The
main application of alumina particles produced by the method according to
the present embodiment is to be used as abrasive grains.

[0036]Alumina particles produced by the method according to the present
embodiment include primary particles preferably each having a hexahedral
shape. For that matter, each primary particle of the alumina particles
particularly preferably has an outer shape similar to a parallelepiped
defined by two square surfaces that face each other and four rectangular
or square surfaces, or a parallelepiped defined by two rhombic surfaces
that face each other and four rectangular or square surfaces. Alumina
particles including primary particles each having a hexahedral shape are
suitable as abrasive grains in an application of polishing an object to
be polished so as to obtain a polished surface with high smoothness and
few defects. The shapes of the primary particles of alumina particles
obtained by calcining boehmite particles usually inherit the almost
shapes of the primary particles of the boehmite particles, so that in
order to obtain alumina particles including primary particles each having
a desired shape, boehmite particles including primary particles each
having the same shape as the desired shape may be calcined.

[0037]The primary particles of alumina particles produced by the method
according to the present embodiment have an average particle size of
preferably 0.6 μm or less, more preferably 0.4 μm or less, further
preferably 0.3 μm or less, and particularly preferably 0.2 μm or
less. In other words, alumina particles produced by the method according
to the present embodiment have an average primary particle size of
preferably 0.6 μm or less, more preferably 0.4 μm or less, further
preferably 0.3 μm or less, and particularly preferably 0.2 μm or
less. The average primary particle size of alumina particles including
primary particles each having a hexahedral shape is defined as an average
length of sides each of which is the longest among three sides extending
from one vertex of each primary particle of the alumina particles.
Alumina particles having an average primary particle size in any of the
above described ranges are suitable as abrasive grains in an application
of polishing an object to be polished so as to obtain a polished surface
with high smoothness and few defects. The value of the average primary
particle size of alumina particles obtained by calcining boehmite
particles usually inherits the almost value of the average primary
particle size of the boehmite particles, so that in order to obtain
alumina particles having a desired average primary particle size,
boehmite particles having the same average primary particle size as the
desired average primary particle size may be calcined.

[0038]In addition, the average primary particle size of alumina particles
produced by the method according to the present embodiment is preferably
0.01 μm or more, more preferably 0.05 μm or more, further
preferably 0.08 μm or more, and particularly preferably 0.1 μm or
more. As the average primary particle size of alumina particles is
larger, the removal rate by the alumina particles against an object to be
polished is enhanced. In this regard, when the average primary particle
size of alumina particles is 0.01 μm or more, or more specifically
0.05 μm or more, 0.08 μm or more, or 0.1 μm or more, the removal
rate is easily enhanced up to a level particularly suitable for practical
use.

[0039]When the primary particles of alumina particles produced by the
method according to the present embodiment each have a hexahedral shape,
the primary particles have an aspect ratio preferably in a range of 1 to
5, more preferably of 1 to 3, further preferably of 1 to 2, and
particularly preferably of 1 to 1.5. The aspect ratio of alumina primary
particles each having a hexahedral shape is defined as an average of
values each obtained by dividing a by c, where the length of the longest
side is represented by a and that of the shortest side is represented by
c among three sides extending from one vertex of each alumina primary
particle. The length of the remaining one side b of each alumina primary
particle is preferably approximately equal to that of the longest side a
of the alumina primary particle. Alumina particles including primary
particles having an aspect ratio of 1 to 5, more specifically 1 to 3, 1
to 2, or 1 to 1.5 are particularly suitable as abrasive grains in an
application of polishing an object to be polished so as to obtain a
polished surface with high smoothness and few defects. The aspect ratio
of the primary particles of alumina particles obtained by calcining
boehmite particles usually inherits the almost aspect ratio of the
primary particles of the boehmite particles, so that in order to obtain
alumina particles including primary particles having a desired aspect
ratio, boehmite particles including primary particles having the same
aspect ratio as the desired aspect ration may be calcined.

[0040]Alumina particles produced by the method according to the present
embodiment have an average secondary particle size preferably in a range
of 0.08 to 2 μm, more preferably 0.15 to 1 μm, and further
preferably 0.2 to 0.7 μm. The average secondary particle size of
alumina particles is equal to the particle size of the last cumulative
particle when the cumulative volume of particles in order from the
smallest particle measured by a laser scattering method becomes 50% or
more of the total volume of all the alumina particles. Alumina particles
having an average secondary particle size in a range of 0.08 to 2 μm,
or more specifically 0.15 to 1 μm, or 0.2 to 0.7 μm are
particularly suitable as abrasive grains in an application of polishing
an object to be polished so as to obtain a surface to be polished with
high smoothness and few defects.

[0041]A value D90/D10 obtained by dividing the 90% particle size (D90) of
alumina particles produced by the method according to the present
embodiment by the 10% particle size (D10) of the same alumina particles,
which is an indicator of the particle size distribution of the alumina
particles, is preferably in a range of 1.2 to 3, more preferably of 1.5
to 2.5, and further preferably of 1.8 to 2.2. The 90% particle size of
alumina particles is equal to the particle size of the last cumulative
particle when the cumulative volume of particles in order from the
smallest particle measured by a laser scattering method becomes 90% or
more of the total volume of all the alumina particles. The 10% particle
size of alumina particles is equal to the particle size of the last
cumulative particle when the cumulative volume of particles in order from
the smallest particle measured by a laser scattering method becomes 10%
or more of the total volume of all the alumina particles. Alumina
particles having a value D90/D10 in a range of 1.2 to 3, or more
specifically 1.5 to 2.5, or 1.8 to 2.2 are particularly suitable as
abrasive grains in an application for polishing an object to be polished
so as to obtain a polished surface with high smoothness and few defects.

[0042]Alumina particles produced by the method according to the present
embodiment may have any crystal form, and may mainly contain any one of,
for instance, a transition alumina such as γ-alumina,
δ-alumina, and θ-alumina, and α-alumina. However, when
high hardness is required, the alumina particles preferably include
α-alumina at least in a part. The alpha ratio of the alumina
particles is preferably 5 to 70%, more preferably 10 to 60%, and
particularly preferably 20 to 50%. The alpha ratio is a value determined
by an X-ray diffraction method based on a comparison result with
corundum. Alumina particles having an alpha ratio in a range of 5 to 70%,
or more specifically 10 to 60%, or 20 to 50% are particularly suitable as
abrasive grains in an application for polishing an object to be polished
so as to obtain a polished surface with high smoothness and few defects.

[0043]In a method for producing alumina particles according to the present
embodiment, boehmite particles obtained through the above described
hydrothermal reaction are used as a starting material. The boehmite
particles are preferably previously cleaned, but are not necessarily
cleaned previously.

[0044]Boehmite particles obtained through the hydrothermal reaction are
dried at first after having been cleaned or without being cleaned. A
typical method for drying boehmite particles includes a flash drying
method.

[0045]The boehmite particles after having been dried are subsequently
calcined, and as a result, alumina particles are obtained.

[0046]The calcination temperature is preferably 500° C. or higher,
more preferably 800° C. or higher, further preferably
1,000° C. or higher, and particularly preferably 1,030° C.
or higher. As the calcination temperature is raised, alumina particles
having a high alpha ratio are easily obtained. In this regard, when the
calcination temperature is 500° C. or higher, or more specifically
800° C. or higher, 1,000° C. or higher, or 1,030° C.
or higher, alumina particles having an alpha ratio of 20% or more is
particularly easily obtained practically.

[0047]In addition, the calcination temperature is preferably 1,200°
C. or lower, more preferably 1,100° C. or lower, and further
preferably 1,070° C. or lower. As the calcination temperature is
lowered, the yield of alumina particles including primary particles each
having a hexahedral shape increases. In this regard, when the calcination
temperature is 1,200° C. or lower, or more specifically
1,100° C. or lower, or 1,070° C. or lower, the yield of
alumina particles including primary particles each having a hexahedral
shape is easily enhanced up to a level particularly suitable for
practical use.

[0048]Alumina particles obtained by calcining the boehmite particles are
then disintegrated. At least some of secondary particles which are formed
of some agglomerating primary particles by the disintegration are divided
into a plurality of particles of which the smallest unit is the primary
particles. A typical method for disintegrating alumina particles includes
a media mill method with the use of balls or beads and a jet mill method.

[0049]The disintegrated alumina particles are preferably subjected to
treatment for removing coarse particles contained in the alumina
particles therefrom. A typical method for removing coarse particles in
alumina particles includes elutriation classification in which the coarse
particles is removed by dispersing the alumina particles in water and
naturally settling down the coarse particles, and a method of removing
the coarse particles by passing the alumina particles through a filter.

[0050]The boehmite particles obtained in this way have primary particles
having almost the same shape as that of the primary particles of the
boehmite particles which are used as a raw material.

[0051]The present embodiment provides the following advantages.

[0052]According to the method for producing boehmite particles of the
present embodiment, boehmite particles having an average primary particle
size of 0.6 μm or less and including primary particles each having a
hexahedral shape are obtained, by subjecting powder of aluminum hydroxide
to hydrothermal reaction together with a nucleation agent. In addition,
according to the method for producing alumina particles of the present
embodiment, alumina particles having an average primary particle size of
0.6 μm or less and including primary particles each having a
hexahedral shape are obtained, by using the boehmite particles as a
starting material, and making the boehmite particles pass through at
least steps of drying, calcination, and disintegration. The alumina
particles are suitable as abrasive grains in an application of polishing
an object to be polished so as to obtain a polished surface with high
smoothness and few defects. The boehmite particles obtained by the method
for producing boehmite particles according to the present embodiment is
useful at least as a raw material for such alumina particles.

[0053]Next, the present invention will be described more specifically with
reference to Examples and Comparative Examples.

EXAMPLE 1

[0054]Slurry was prepared by dispersing 80 g of gibbsite powder and 0.4 g
of alumina sol into 720 g of pure water. The obtained slurry was stirred
at a rotation speed of 8,000 rpm for 20 minutes, and then was transferred
into an autoclave. The autoclave was heated to the temperature of
200° C. at a heating rate of 1.5° C./minute, and was held
at 200° C. for 4 hours. Afterwards, the autoclave was naturally
cooled to room temperature, and then, the slurry which had finished the
hydrothermal reaction and contained boehmite particles synthesized
through the reaction was taken out from the autoclave. The boehmite
particles were cleaned by collecting, as filtrate, a water phase in the
slurry which had finished the reaction, while introducing pure water into
the slurry, until the electrical conductivity of the collected filtrate
reached 20 μS/cm or lower.

EXAMPLE 2

[0055]A mixture of 80 g of gibbsite powder and 720 g of pure water was
subjected to cross flow filtration with the use of an ultrafiltration
membrane to adjust the electrical conductivity to 50 μS/cm.
Afterwards, 0.4 g of alumina sol was added to the mixture, and nitric
acid was further added thereto to adjust the pH to 6.0. Except that the
slurry obtained in this way was used in place of the slurry in Example 1,
boehmite particles were produced according to similar procedures to those
in Example 1.

EXAMPLE 3

[0056]A mixture of 80 g of gibbsite powder and 720 g of pure water was
subjected to cross flow filtration with the use of an ultrafiltration
membrane to adjust the electrical conductivity to 50 μS/cm.
Afterwards, 0.4 g of alumina sol was added to the mixture, and sodium
nitrate was further added thereto until the electrical conductivity
reached 2,400 μS/cm. Except that the slurry obtained in this way was
used in place of the slurry in Example 1, boehmite particles were
produced according to similar procedures to those in Example 1.

EXAMPLE 4

[0057]A mixture of 80 g of gibbsite powder and 720 g of pure water was
subjected to cross flow filtration with the use of an ultrafiltration
membrane to adjust the electrical conductivity to 50 μS/cm.
Afterwards, 1.6 g of alumina sol was added to the mixture, and nitric
acid was further added thereto to adjust the pH to 4.8. Except that the
slurry obtained in this way was used in place of the slurry in Example 1,
boehmite particles were produced according to similar procedures to those
in Example 1.

EXAMPLE 5

[0058]A mixture of 80 g of gibbsite powder and 720 g of pure water was
subjected to cross flow filtration with the use of an ultrafiltration
membrane to adjust the electrical conductivity to 50 μS/cm.
Afterwards, 1.6 g of alumina sol was added to the mixture, and nitric
acid was further added thereto to adjust the pH to 4.0. Except that the
slurry obtained in this way was used in place of the slurry in Example 1,
boehmite particles were produced according to similar procedures to those
in Example 1.

[0059]Physical properties (average primary particle size, average
secondary particle size, and aspect ratio) of the boehmite particles
which were obtained in the above described Examples 1 to 5 are shown in
Table 1 together with the values of the electrical conductivity and pH of
the slurry prior to being subjected to the hydrothermal reaction.

[0060]Slurry was prepared by dispersing 120 g of gibbsite powder and 1.2
mg of alumina sol into 680 g of pure water. The slurry was stirred at a
rotation speed of 8,000 rpm for 20 minutes, and then was transferred into
an autoclave. The autoclave was heated to the temperature of 200°
C. at a heating rate of 1.5° C./minute, and was held at
200° C. for 4 hours. Afterwards, the autoclave was naturally
cooled to room temperature, and then, the slurry which had finished the
hydrothermal reaction and contained boehmite particles synthesized
through the reaction was taken out from the autoclave. The boehmite
particles were cleaned by collecting, as filtrate, a water phase in the
slurry which had finished the reaction, while introducing pure water into
the slurry, until the electrical conductivity of the collected filtrate
reached 20 μS/cm or lower. The boehmite particles after having been
cleaned were flash-dried, and then were calcined in a calcination furnace
to obtain alumina particles. In the calcining step, the calcination
furnace was heated to the temperature of 1,050° C. at a heating
rate of 2° C./minute, and was held at 1,050° C. for 3
hours. The obtained alumina particles were disintegrated by using a jet
mill at a disintegration pressure of 0.6 MPa. Finally, coarse particles
having sizes of 2 μm or more were removed by elutriation
classification.

EXAMPLE 7

[0061]Except that the amount of alumina sol to be added was changed from
1.2 mg to 4.8 mg, alumina particles were produced according to similar
procedures to those in Example 6.

EXAMPLE 8

[0062]Except that the amount of alumina sol to be added was changed from
1.2 mg to 24 mg, alumina particles were produced according to similar
procedures to those in Example 6.

EXAMPLE 9

[0063]Except that the amount of alumina sol to be added was changed from
1.2 mg to 60 mg, alumina particles were produced according to similar
procedures to those in Example 6.

EXAMPLES 10 AND 11

[0064]Except that the amount of alumina sol to be added was changed from
1.2 mg to 240 mg, and the electrical conductivity and pH of the slurry
prior to being subjected to hydrothermal reaction were adjusted to 150
μS/cm and 7.0, respectively, through cross flow filtration with the
use of an ultrafiltration membrane, and the calcination temperature was
changed from 1050° C., alumina particles were produced according
to similar procedures to those in Example 6. The calcination temperature
was 1030° C. in Example 10, and 1040° C. in Example 11.

EXAMPLE 12

[0065]Except that the amount of alumina sol to be added was changed from
1.2 mg to 240 mg and the electrical conductivity and pH of the slurry
prior to being subjected to hydrothermal reaction were adjusted to 150
μS/cm and 7.0, respectively, through cross flow filtration with the
use of an ultrafiltration membrane, alumina particles were produced
according to similar procedures to those in Example 6.

EXAMPLE 13

[0066]Except that 120 mg of aluminum nitrate nonahydrate was used in place
of the alumina sol, alumina particles were produced according to similar
procedures to those in Example 6.

COMPARATIVE EXAMPLE 1

[0067]Except that 12 mg of aluminum nitrate nonahydrate was used in place
of the alumina sol, alumina particles were produced according to similar
procedures to those in Example 6.

COMPARATIVE EXAMPLE 2

[0068]Except that 240 mg of aluminum nitrate nonahydrate was used in place
of the alumina sol, alumina particles were produced according to similar
procedures to those in Example 6.

COMPARATIVE EXAMPLE 3

[0069]Except that 120 mg of ferric chloride was used in place of the
alumina sol, alumina particles were produced according to similar
procedures to those in Example 6.

COMPARATIVE EXAMPLE 4

[0070]Except that the addition of the alumina sol was omitted, alumina
particles were produced according to similar procedures to those in
Example 6.

COMPARATIVE EXAMPLE 5

[0071]Slurry having alumina particles dispersed in pure water was prepared
by charging 1.2 kg of alumina particles having an average particle size
of 50 μm into 1.5 kg of pure water together with a dispersing agent.
This slurry was charged into a pot of a pot mill together with 6 kg of
alumina balls having a diameter of 8 mm, and the alumina particles were
pulverized by rotating the pot mill at a rotation speed of 70 rpm for 30
hours.

COMPARATIVE EXAMPLE 6

[0072]Except that a rotation period of time of a pot mill was changed from
30 hours to 60 hours, alumina particles having an average particle size
of 50 μm were pulverized according to similar procedures to those in
Comparative Example 5.

[0073]The type and the content of a nucleation agent used in each of
Examples 6 to 13 and Comparative Examples 1 to 6 are shown in Table 2.
The results of having measured the aspect ratio, the alpha ratio, the
average primary particle size, and the average secondary particle size of
the alumina particles which were finally obtained in each of Examples 6
to 13 and Comparative Examples 1 to 6 are also shown in Table 2.

[0074]A polishing composition was prepared by mixing the alumina particles
which were finally obtained in each of Examples 6 to 13 and Comparative
Examples 1 to 6 in water together with aluminum nitrate nonahydrate
(polishing accelerator), tetrasodium glutamate diacetate (cleaning
accelerator), and hydrogen peroxide (oxidizing agent). In the case of any
polishing composition, the content of alumina particles was 1.5% by mass,
the content of aluminum nitrate nonahydrate was 3 g/L, the content of
tetrasodium glutamate diacetate was 0.3 g/L, and the content of hydrogen
peroxide was 13 g/L.

[0075]The surface of an electroless nickel-phosphorus plated substrate for
a magnetic disk having a diameter of 3.5 inches (≈95 mm) was
polished by using each of the polishing compositions in conditions shown
in Table 3, and a polishing rate at this time was determined on the basis
of the difference between weights of the substrate before and after
polishing. The result is shown in the column of "polishing rate" in Table
2.

[0076]The number of scratches was measured which existed on the surface of
the substrate that was polished by using each of the polishing
compositions and then was rinsed with pure water. Specifically, the
number of the scratches was visually measured while irradiating the
surface of each substrate with a light emitted from a surface inspection
lamp "F100Z" made by FUNAKOSHI YAKUHIN KK. The measurement results were
evaluated to be excellent (∘∘) when the number of
the measured scratches was less than 25, to be good (∘) when
the number was 25 or more but less than 50, to be slightly poor (×)
when the number was 50 or more but less than 75, to be poor
(××) when the number was 75 or more but less than 100, and to
be extremely poor (×××) when the number was 100 or
more. The evaluation result is shown in the column of "number of
scratches" in Table 2.

[0077]The surface of the substrate was visually observed under a
fluorescent lamp, after being polished by using each of the polishing
compositions and then rinsed with pure water, and the presence or absence
of the deposition of alumina particles on the surface of the substrate
was checked. The observation results were evaluated to be good
(∘) when the deposition of alumina particles was not
confirmed, and to be poor (×) when the deposition of alumina
particles was confirmed. The evaluation result is shown in the column of
"ease of washing off" in Table 2.

[0078]From the results shown in Table 1, it was found that the values of
the electrical conductivity and pH of the slurry before being subjected
to hydrothermal reaction exerted an influence upon physical properties of
the obtained boehmite particles.

[0079]From the results shown in Table 2, it was found that when alumina
sol or aluminum nitrate was used as a nucleation agent, and particularly
when alumina sol was used, alumina particles having a comparatively small
average primary particle size were easily obtained. The alumina particles
which were obtained in Comparative Examples 5 and 6 had a comparatively
small average primary particle size, but the evaluation relating to
scratches and the evaluation relating to washing were not adequate. This
is considered to be because each primary particle of the alumina
particles does not have a hexahedral shape.